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How Tstats work?

I have a very basic question but one that I have been unable to find a clear answer to.

I do understand the difference in principle between "ambient" and "radiant" temperatures. And how and why they can differ. Thanks to an article that I found on this website.

I have a simple Honeywell Round Non-programmable Heat Only Tstat in my 100+ year old home, a CT87K. Is the sensor that makes the decision that the steam boiler should be turned on based on ambient or radiant temp or a combo of both?

If I were to move to a simple Digital model - is the on/off decision based on the same thing - ambient, radiant, or both? For example, the RTH5160, but I think these low end digitals all probably work the same way.

A second question - on the digital display of most Tstats - there are two temps - the "set" temp and another temp. Is this other temp simply the ambient temperature (similar to the lower dial on my Round CT87K) or is it the temp that triggers the on/off decision.

Believe it or not, I have not been able to get a straight answer from Honeywell customer support.

Many thanks. Cathy M.


  • Jamie Hall
    Jamie Hall Member Posts: 23,654
    You seem to have started two different foxes here, Cathy, but they're pretty much the same -- so I'll chase this one.

    And the answer is they really sense ambient. Almost all thermostats today -- your round C87K on up to the fanciest ones -- have a small device inside called a "thermistor" which is simply a bit of electronic wizardry which changes resistance as the temperature changes. It's really sensing the temperature of the circuit board on which it is mounted (or whatever else it's attached to) and that in turn is determined mostly by the ambient temperature -- that is, the space temperature. However... if you were to shine a heat source on it, such as put it in the sun, the board or whatever will warm up, and that would be a result of radiant energy which will make it warmer, and thus you could call that radiant temperature.

    Now there is more electronic wizardry inside, and some of it will display the resistance of that thermistor to you, converted conveniently into a temperature equivalent (except I believe -- but am not sure -- that your CT87K has a real "bimetal" thermometer for the display temperature -- haven't taken one apart). It might display on by moving a needle on a scale, or more often by simply lighting up digits. Yet more wizardry will compare that resistance to a resistance which you select -- either directly, by turning a dial, or by pressing buttons to display your set temperature and then if the thermistor is sensing a temperature lower than what you want, it will turn on your heating system.

    In fancier thermostats the relationship between the sensed temperature, the set temperature, and on or off is a little more complicated, since the fancier ones (including the CT87K) will take into account how long it takes your system to heat up -- or how long it takes it to cool off when it turns off -- to try to "anticipate" how your space will react, which helps to keep a more constant space temperature. As a result it is quite common to note that the temperature displayed when the system turns on or off is slightly different from the temperature you set -- but they are both looking at the same thing: the temperature of the device, which is usually just ambient at the device's location.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited November 2019
    Now that's an answer, right? Send it to Honeywell (and Honeywell should cut @Jamie Hall a check).

    There was an error rendering this rich post.

  • Intplm.
    Intplm. Member Posts: 2,084
    @Jamie Hall Excellent !! Answer, explanation.
    A true Yoda answer.
  • Reillydog
    Reillydog Member Posts: 4

    Now that's an answer, right? Send it to Honeywell (and Honeywell should cut @Jamie Hall a check).

    I agree. Thanks, Jamie.

    And yes, my CT87K does have an anticipatory fx - whereby it turns on the heat below my "set" temp (about 2 degrees) and shuts it off before it reaches the "set" temp, so as not to overshoot the "set" temp.

    So grateful for your help. Cathy M.
  • D107
    D107 Member Posts: 1,860
    edited November 2019
    Just to briefly follow up on Cathy's excellent question: So a t-stat like the Honeywell T6 Pro, which has 12 different levels of what they call 'heating cycle rate' which I'm hoping is another way of saying 'anticipator', that means that one can choose to have the heat come on e.g. at .5º below set point and cut out at setpoint or slightly before or after?

    If one has cast iron rads then temp might rise after shutoff but I've often felt with an old White Rodgers stat I have that if the stat is set right the ambient temp reading will almost always be the same as the target point. Cheaper t-stats without all the bells and whistles will let temp get down to a degree below setpoint before calling for heat. Just looking at it makes you feel colder.

    And the T6 has 'adaptive intelligent recovery' and not sure how that works with cycle rate--especially if you basically have no setback and keep the ambient temp constant.